Egypt's vice president has stepped down as the country voted in a referendum that is expected to approve a new constitution that will strip him of his role.

Authorities extended voting by four hours in the second and decisive round of the plebiscite on an Islamist-drafted constitution that the opposition has criticised as divisive and likely to cause more unrest.

Mahmoud Mekki said he wanted to quit last month but stayed on to help President Mohammed Morsi tackle a crisis that blew up when the Islamist leader assumed wide powers.

In his resignation statement on state television, Mr Mekki said: "I have realised a while ago that the nature of politics don't suit my professional genesis as a judge."

Mr Mekki said he wanted to quit last month

Mr Mekki, 58, was a respected judge before being elevated to the post of vice president by Mr Morsi in August. He led the judicial opposition to the ousted president Hosni Mubarak but resisted calls to run for power himself.

His resignation came shortly before Central Bank governor Farouk el Okadah announced he too was leaving his post.

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Gallery: Egypt Protests 15th Dec

A supporter of the opposition to Egyptian President Mohamed Morsi plays football next to a Republican Guard tank deployed outside the presidential palace in Cairo.

State television reported his deputy, Hisham Ramez, was the most likely candidate to fill his post.

Both the former governor and his deputy helped steer the central bank during last year's uprising that ousted former President Hosni Mubarak and worked to keep the Egyptian currency relatively stable despite the political turmoil.

Mr Morsi sparked widespread unrest when he assumed sweeping new powers last month in a move that put him at odds with the judiciary and much of the public.

His opponents said he was acting like the "new pharaoh" and protests soon escalated to violent clashes between Mr Morsi's supporters and his detractors.

He eventually backed down and gave up the powers but instead has persisted with his attempt to push through a hastily drawn up constitution that the opposition claims does not represent all Egyptians.

Critics say the document is divisive as it favours Mr Morsi's Islamist allies and ignores the rights of Christians and women.

Egypt has been in turmoil for more than four weeks and the latest violent clashes took place on Friday, when supporters and opponents of the president clashed in the Mediterranean port city of Alexandria. Dozens were hurt.

The first phase on December 15 produced a "yes" majority of about 56% with a turnout of some 32%, according to unofficial results.

The opposition has alleged abuses, and called for a re-vote of the first stage.

But the committee overseeing the referendum said their investigation had turned up no major irregularities.

If the constitution is adopted, a parliamentary election will be held in about two months.